The present invention relates to the field of borehole drilling, and more particularly to that of drilling relatively large diameter, substantially vertical boreholes. More particularly still, the present invention relates to the drilling of boreholes used to support street lighting, power poles, stadium lighting, or similar structures, or for constructing caissons or the like, such as those used to hold down a stadium roof or the like.
In order to provide adequate support for light poles, power poles, or the like, or to construct caissons such as those used to anchor the roof of a stadium or other structure, relatively large diameter boreholes must be drilled into the earth. Such boreholes typically can be from about 24 inches to about 72 inches in diameter. When drilled to the proper depth for the particular application, the pole or other structure is placed in the borehole, and the hole is backfilled with rock, soil or other material, or cemented in, to secure the pole or the like properly in the borehole, or to construct the caisson.
In the past, drilling of such large diameter boreholes has been relatively slow and inefficient, requiring rigs having relatively large torque and horsepower capabilities. One type of bit used in the past for drilling of this kind is the core barrel bit, which essentially is a piece of pipe with teeth disposed around its circumferential periphery. With this type of bit, as it rotates the teeth cut a circle around a core, which initially remains substantially intact within the bit. After the bit has traveled an incremental distance downhole, which may be about the same as the axial height of the bit, the core disposed inside the bit must be broken off from the earth or rock below before it can be removed from the hole. Sometimes the operators experience difficulty in breaking off the core; often special wedges or the like must be employed for this purpose. Breaking off the cores also tends to put undue stress on the drilling rig, as well as on the bits and on the bolts or like fasteners which secure the bits to the rig. Such stress can result in damage to the bits or the drilling rig, and can even lead to loss of the bits downhole.
Another type of bit used in the past for drilling relatively large diameter boreholes is the rock auger, which is referred to in the art as a righted bit. The term "flighted" means that the bit has a plurality of axially spaced apart surfaces or levels which may engage the borehole. The flights of the rock auger are thus the turns of the auger screw, which in cross section appear to be stacked, spaced apart, generally horizontal surfaces. Some of the rock augers used in the past have included a plurality of teeth welded in two opposed rows below their lower faces and extending to their edges. The teeth used in these prior art rock augers have typically been spaced at intervals of about 5 inches. It is believed that these two rows of teeth found in typical prior art rock augers grind, rather than cut or fracture, the rock during drilling. Grinding the rock rather than cutting or fracturing it is a relatively slow drilling process. This is a significant disadvantage for the rock auger-type bits. Another disadvantage results from the rather large (about 5 inches) spacing between the teeth of these prior art bits. The large spacing between teeth permits the rock at the borehole bottom to engage and wear both the tooth pockets and the bottom of the flighting of the bit. In addition, the outer peripheral edges of the flights of the auger tend to engage and bind or stick against the borehole walls, thus creating high torque on, and sometimes breaking, the drilling rigs or components thereof. Moreover, flighted bits such as the rock auger tend to walk on top of the rock and shift in the hole. Furthermore, sometimes it is difficult to lift a flighted bit out of the hole after accumulating a large volume of loose material atop the bit. The loose material rests on, and weighs down on, the upper surfaces of the flights, and when the bit is lifted up, so is all the loose material sitting on top of the bit. Lifting the loose material sitting on top of the bit requires additional horsepower from the rig, and places undue stress on the rig and on the bits. Moreover, when only 2 drilling tooth-edges are used, such as in the typical rock auger referred to above, and when several hard earth formations are interspersed with several soft ones, the 2 edges tend to ride up and down on the formations and subject the drilling rig to excessive shaking. This also varies the torque required from the rig, say from 30,000 foot-pounds down to 5,000 foot-pounds, and then back to 30,000 foot-pounds, and then back to 5,000 foot-pounds, etc. Besides unduly shaking the rig, this cycling of torque has the potential of severely damaging or breaking the drive shafts or other components of the rig.